In recent years, nonaqueous electrolyte secondary batteries using an alloy capable of storing and releasing lithium, metallic lithium or a carbon material as the negative active material and a lithium transition metal complex oxide represented by the chemical formula: LiMO2 (M indicates a transition metal) as the positive active material have been noted as high-energy-density batteries.
A representing example of the lithium transition metal complex oxide is a lithium cobalt complex oxide (LiCoO2), which has been already put to practical use as the positive active material for nonaqueous electrolyte secondary batteries.
For nonaqueous electrolyte secondary batteries using a lithium transition metal complex oxide, such as lithium cobaltate, as the positive active material and a carbon material or the like as the negative active material, an end-of-charge voltage is generally prescribed at 4.1-4.2 V. In this case, the active material of the positive electrode utilizes only 50-60% of its theoretical capacity. Accordingly, if the end-of-charge voltage is increased to a higher level, a capacity (utilization factor) of the positive electrode may be improved so that the battery capacity and energy density are increased.
However, the higher end-of-charge voltage renders LiCoO2 more prone to experience structural degradation and increases a tendency of an electrolyte solution to decompose on a surface of the positive electrode. As a result, the battery in this case experiences marked deterioration with charge-discharge cycles, compared to the conventional case where the end-of-charge voltage is set at 4.1-4.2 V, which has been a problem.
Among the lithium transition metal complex oxides represented by LiMO2 (M indicates a transition metal), those containing Mn and Ni as a transition metal, as well as materials containing all of the three transition metal elements Mn, Ni and Co, have been extensively studied (for example, Patent Literatures 1 and 2 and Non-Patent literature 1).
Among those lithium transition metal complex oxides containing Mn, Ni and Co, a compound having the same composition of Mn and Ni is reported as showing a uniquely high thermal stability even in a charged state (high oxidation state) (for example, Non-Patent Literature 2). It is also reported that the complex oxide having substantially the same composition of Ni and Mn has a voltage of approximately 4 V, as comparable to that of LiCoO2, and exhibits a high capacity and a superior charge/discharge efficiency (Patent Literature 3).
Batteries using such a lithium transition metal complex oxide containing Mn, Ni and Co and having a layered structure as the positive active material, because of their high thermal stability at charged state, can be expected to achieve a marked reliability improvement even when the end-of-charge voltage is elevated to thereby increase the depth of charge at the positive electrode.
However, after the study on the battery using the lithium transition metal complex oxide containing Mn, Ni and Co as the positive active material, the inventors of this application have found that the higher end-of-charge voltage renders the positive active material more prone to experience structural degradation and increases the occurrence of decomposition of an electrolyte solution on a surface of the positive electrode and, as a result, the battery in this case shows marked capacity decline with charge-discharge cycles, compared to the conventional case where the end-of-charge voltage is set at 4.1-4.2 V.    Patent Literature 1: Patent Registration No. 2,561,556    Patent Literature 2: Patent Registration No. 3,244,314    Patent Literature 3: Patent Laying-Open No. 2002-42,813    Non-Patent Literature 1: Journal of Power Sources, 90(2000), 176-181    Non-Patent Literature 2: Electrochemical and Solid-State Letters, 4(12), A200-A203(2001)    Non-Patent Literature 3: 42nd Battery Symposium in Japan, Lecture Summary, pp 50-51